POMC processing is frequently altered in ACTH-secreting nonpituitary tumors in which intermediate lobe-like peptides such as corticotropin-like intermediate lobe peptide (CLIP) are occasionally generated. In rodent pituitaries, the exclusive presence of prohormone convertase PC2 in the melanotrophs of the intermediate lobe is responsible for the specific conversion of ACTH to alpha MSH and CLIP, by contrast with corticotrophs of the anterior lobe, which do not contain PC2 and, therefore, only produce ACTH. The goal of our study was to look for PC2 expression in ACTH-secreting nonpituitary tumors in man. Using Northern blot analysis, PC2 transcripts were detected in five nonpituitary tumors that contained large proportions of CLIP (from 40-95% of the total C-terminal immunoreactive ACTH). A predominant PC2 messenger ribonucleic acid migrated with an apparent mol wt of 5 kilobases, and a minor signal at 3 kilobases was also detected. No PC2 messenger ribonucleic acid could be detected in the small cell carcinoma of the lung-derived DMS-79 human cell line, which produces unprocessed POMC, or in three pituitary tumors responsible for Cushing's disease or Nelson's syndrome, which produced intact ACTH, but no CLIP. These data strongly suggest that, as in rodents, PC2 is responsible for the production of smaller POMC end products, such as CLIP, frequently observed in ACTH-secreting nonpituitary tumors in man.
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